Xtol was created in 1996, making it the last really new developer. My understanding is that one of the design goals was to make a developer that was less toxic, and that ascorbic acid was part of it. I wish they still sold it in 1 liter increments. The current package makes 5 liters which means its hard for me to use before it goes bad.
Can you separate the two powder packets by weight into 5 parts?
Yes. I like to weight it out on a digital scale and make 2.5 liter batches. Seal the bag of unused powder with tape.
Generally it's better to avoid doing this, because you're unlikely to get a even split of the ingredients in each packet.
It has lasted 2 years for me if done with distilled water and stored in soda bottles with no air.